The U.S. population of dual language learners (DLLs), or children who are exposed to and learning through two languages, has grown rapidly in the last decade. DLLs from low-income families lag significantly behind their monolingual English-speaking peers on school readiness measures at kindergarten entry, and the achievement gap continues to widen with age. Despite the critical role of socio-emotional development (SED) in children's long-term health and achievement, significant gap exists in our knowledge on how bilingual experience shapes SED. By integrating the assessment methods on language, executive functions, and SED, this R01 takes an interdisciplinary approach to study the reciprocal relations between bilingual (English and heritage language) development and SED in a longitudinal study. We will follow 400 DLLs (50% females, initially 3-4 years of age) from low-income Spanish-speaking Mexican American families (N = 200) and Cantonese-speaking Chinese American families (N = 200) annually for three years (T1 and T2 in preschool, T3 in kindergarten). Three waves of multi-method and multi-context data on children's English and heritage language proficiency, executive functions, SED, and quality of parent-child and teacher-child relationships will be collected to address the following specific aims: 1) Investigate the contributions of early English and heritage language development to DLLs' later SED, and test executive functions as a mediating mechanism; 2) Examine the contributions of early bilingual development to DLLs' parent-child and teacher-child relationships, and test relationship quality as mediators between language and SED; 3) Examine cultural differences in DLLs' social relationships, and test whether culture moderates the links between language and relationships; and 4) Test whether other child, family, and school factors moderate the relations between language and SED. The proposed research will contribute to the scientific knowledge on the linking of development across domains (language, executive function, and SED), and have implications for early childhood education policy, assessment, curriculum development, teacher training, and family engagement practice on DLLs.